A 15-year-old girl was sexually assaulted outside the Truebright Science Academy in North Philadelphia at about 7:40 a.m. on Monday. Police said a heavyset 45- to 55-year-old black man grabbed the girl from behind and pinned her against a wall outside the school, on Broad Street near McFerran, and then chased her inside. The attacker exposed himself but was chased away by a school dean before he could do any more harm. The victim, a Truebright student, told police the man had followed her from Broad and Erie Avenue. Police said the attacker has a dark complexion, light bear and is balding with gray hair. He wore a dirty white t-shirt, black hooded jacket and light blue pants that were rolled up to his ankles. Tipsters can call 215-685-3251.